mkvtoolnix: updated to 56.0.0

Version 56.0.0 "Strasbourg / St. Denis" 2021-04-05

New features and enhancements

* mkvmerge, mkvpropedit: tags: the programs will no longer write tag elements
  that are mandatory and set to their default value (e.g. "tag language" set
  to `und` = undetermined).
* mkvmerge, mkvpropedit, MKVToolNix GUI chapter editor: chapters: the programs
  will no longer write chapter elements that are mandatory and set to their
  default value (e.g. "chapter language" set to `eng` = English or "Chapter
  flag enabled" = 1).
* mkvextract: chapters: mkvextract will no longer add a `ChapterLanguage`
  element set to `eng` to the generated XML content if the source file doesn't
  contain such an element.
* MKVToolNix GUI: multiplexer: when using the "tab widget below the files &
  tracks" layout option for the track properties the elements in the "general
  options" tab will use six rows à four columns instead of twelve rows à two
  columns, greatly reducing the required height.

Bug fixes

* mkvmerge: AAC reader: fixed mkvmerge aborting to read AAC files bigger than
  2 GB with a message about not being able to allocate memory.
* mkvmerge: chapters: `ChapLanguageIETF` elements were still created when the
  option `--disable-language-ietf` was given after an option leading to the
  creation of chapters. Now the position doesn't matter anymore.
* mkvmerge: chapters: when the option `--disable-language-ietf` is used,
  `ChapLanguageIETF` elements won't be written even when sources are read
  (Matroska files or XML chapters files) that do contain those elements.
* mkvmerge: tags: when the option `--disable-language-ietf` is used,
  `TagLanguageIETF` elements won't be written.
* mkvmerge: tags: mkvmerge will no longer write language elements for the
  track statistics tags it creates, making the effective language
  "undetermined" due to `und` being the default value for the legacy tag
  language element.
* mkvmerge: tags: the XML tag parser will now validate all `<Simple>` children
  of `<Tag>` elements, not just the first for each `<Tag>`.
* mkvmerge: tags: the XML tag parser now uses the spec-compliant `und`
  ("undetermined") as the default language for `<Simple>` tags instead of
  `eng`.
* mkvmerge, mkvpropedit: tags: mkvmerge will no longer set the "target type"
  for track statistics tags (earlier it used `MOVIE`). The "target type value"
  will still be set to `50`.
* mkvmerge, mkvextract: HEVC/H.265: both programs will now normalize the
  placement of VPS, SPS and PPS NALUs. Each key frame is prefixed with exactly
  one copy of the currently active parameter sets. This fixes certain classes
  of bugs related to splitting/appending.
* mkvinfo: when compiled with newer versions of the `fmt` library, certain
  numbers were not output correctly (e.g. a track's audio sampling frequency
  of 48000 might be output as 48).
* MKVToolNix GUI: multiplexer: deriving the track language from the file name
  will match languages case insensitively again (like versions prior to
  v55).
* MKVToolNix GUI: multiplexer: under certain circumstances keyboard shortcuts
  such as `Ctrl+W` for closing the active tab or `Ctrl+R` for starting to
  multiplex ceased to work until the user did certain other things (such as
  switching to a different tool & back to the multiplexer or opening the
  multiplexer menu). This started with v54.
* MKVToolNix GUI: chapter editor: the editor will no longer create empty
  `ChapterCountry` elements when adding a chapter name with no default country
  selected in the preferences.

Build system changes

* The bundled `fmt` library was updated to v7.1.3.

(adam)